1. Field of the Invention
This invention relates to method and apparatus for determining the length of an elongated tubular member such as a length of drill pipe or well casing, and in particular to electro-acoustic measuring apparatus.
2. Description of the Prior Art
It is common practice in the petroleum production business and in other industries to stack various lengths of pipe in the same storage location. It is important for a number of reasons to determine the length of each pipe within a close tolerance. For example, in the petroleum production industry, various categories of tubing, for example Range I tubing (17'-27') may be stacked together with Range II tubing (27'-37') and Range III tubing (37'-47'). It is common practice to inspect such tubing by non-destructive testing means, and to apply a protective coating to the tubing. The service costs for these procedures are charged by the foot. Thus, on a cost basis, it is desirable to be able to accurately determine the total length of tubing involved before it is released.
Present pipe measuring procedures involve the use of a forklift for physically removing and handling lengths of pipe, and three or more persons to perform the actual measurement of each pipe with a steel tape measure. Two persons operate the tape measure, with one person recording the length information and one forklift operator moving the pipe. The best accuracy presently obtainable by such methods is 5/8"-3/4" for 47' pipe lengths. However, such procedures are prohibitively time consuming and expensive because of the personnel and equipment required, and involve risk of personal injury.
Recently, electro-acoustic apparatus has been developed for accurately measuring the lengths of pipe. Such apparatus commonly employs electronic circuits and electro-acoustic transducers for accurately determing the length of the pipe. However, such instruments have not met with widespread acceptance because of the lack of repeatability in the measurements. This is believed to be caused in part by naturally occuring variations in ambient temperature, atmospheric pressure and air density. Improvements to such instruments have included circuits for sensing temperature and pressure for accurately determining the speed of sound so that accurate, repeatable measurement can be made. Because of this added circuitry, such instruments are relatively complex and expensive. There is a continuing interest in simplifying the equipment for carrying out measurements by electro-acoustic means, and in particular to improving the accuracy and repeatability of measurements made with such instruments.